In a scanning optical apparatus used for an electrophotographic image forming apparatus, a light beam emitted from a light source is converted into a spot-like image and focused on a target surface to be scanned, such as a peripheral surface of a photoconductor drum. The light beam rapidly sweeps the surface of the photoconductor drum in an axial direction (main scanning direction) thereof. The scanning optical apparatus includes a light deflector for deflecting the light beam in the main scanning direction. The scanning optical apparatus further includes an incident optical system located upstream of the light deflector, and a scanning optical system located downstream of the light deflector. The incident optical system converges the light beam in the sub-scanning direction to bring the same to a focus in the proximity of the light deflector while rendering the light beam nearly parallel in the main scanning direction. Meanwhile, the scanning optical system focuses the light beam deflected by the light deflector onto the target surface to form a spot-like image.
However, if the environmental temperature at which the scanning optical system is used varies, dimensions of parts or performances of optical elements will change, with the result that the focal point shifts (i.e., focus movement occurs) in the front-and-rear direction (in the traveling direction of the light beam) from the target surface. This is particularly serious if a plastic lens is used in the incident optical system for the purpose of reducing the manufacturing cost.
To address this problem, a conventional scanning optical apparatus includes an incident optical system provided with a refracting unit (refracting surface) and a diffraction unit (diffraction surface) so that a focus movement (i.e., focal plane shift) caused by temperature change can be suppressed (temperature change can be compensated). Further, it is known that satisfying the following conditions is preferable to suppress the focus movement: a power of the refraction unit is φr, a power of the diffraction unit is φd, and 0.6<φr/φd<0.9.